1. Field of the Invention
The present invention relates to a compact, high-performance tuning apparatus, which receives a radio wave or the like and to an improvement on a method for receiving in a radio-controlled timepiece using the above-noted tuning apparatus.
2. Background Art
In the past, the electronic tuning apparatus used in an electronic tuner of a receiver or in the oscillator of a transmitter generally used a variable-capacitance diode, whereby a voltage applied to the variable-capacitance diode is controlled so as to vary its equivalent electrostatic capacitance, thereby varying the tuned frequency. Because this method, in addition to using a compact, low-cost variable-capacitance diode, enables easy variation of the tuned frequency by controlling the applied voltage, it has an advantage in enabling compact and low-cost implementation of tuners or transmitting circuits. However, because a variable-capacitance diode makes use of the depletion layer of a semiconductor PN junction, a leakage current exists, making it impossible to achieve a tuning circuit with a high Q value.
Also, because the Q value changes with a change in the electrostatic capacitance, it is difficult to achieve a stable tuning circuit. Additionally, because of the limited variable capacitance range of a variable-capacitance diode, it is difficult to vary the tuned frequency over a wide range.
To solve these problems, there was a proposal, such as in the claims and FIG. 3 of the specification of Japanese Unexamined Patent publication (KOKAI) No. 57-99787, in which a plurality of variable-capacitance diodes capable of taking either of a pair of binary conditions in capacitance, i.e., a maximum capacitance value and a minimum capacitance value, are formed on a semiconductor substrate, and a bias voltage with respect to each variable-capacitance diode is on/off switched by a switching element on the same semiconductor substrate, so as to change the equivalent electrostatic capacitance value.
According to this proposal, because switching elements are used to switch a plurality of variable-capacitance diodes, it is possible to achieve a wide range of variable capacitance, thereby enabling a wide variable frequency range. Additionally, because there is little change in the Q value with a change in electrostatic capacitance, it is possible to achieve a tuning circuit with some degree of stability.
However, because the plurality of variable-capacitance diodes formed on the semiconductor substrate make use of the depletion region of a PN junction, as noted above, a leakage current exists, so that it is not possible to increase the Q value of the tuning circuit beyond a certain amount, and difficult to achieve a stable tuning circuit. Additionally, in order to minimize the capacitance of the variable-capacitance diodes, it is necessary to apply a bias voltage that is somewhat high, and the generation of the bias voltage requires a negative power supply, this representing an important factor in increasing the number of components and the cost and the like of the tuning circuit. Also, although a plurality of PN junctions are formed on one semiconductor substrate to form variable-capacitance diodes, in order to prevent electrical interaction between adjacent variable-capacitance diodes and suppress variation of Q value, it is necessary to form an insulating region between each of the individual variable-capacitance diodes so as to electrically separate them, thereby increasing the number of semiconductor substrate manufacturing process steps, lowering the yield, and increasing the cost.
Accordingly, it is an object of the present invention to solve the above-noted problems by providing a tuning apparatus which uses a combination of semiconductor switches with capacitors having an extremely small leakage current, thereby providing superior stability and a wide frequency tuning range, making the tuning apparatus suitable for use in a compact radio-controlled timepiece, and by providing a radio-controlled timepiece using this tuning apparatus.